Influence of Spin-Dependent Polaron Pair Charge Recombination Rates on Singlet Exciton Yields of Fluorescent Organic Light-Emitting Diode Materials

Hot exciton materials have been recently studied toward improving the efficiency of fluorescent organic light-emitting diodes (OLEDs). The improvement is achieved by harvesting triplet excitons through high-lying reverse intersystem crossing (hRISC), and for its success, it is necessary to suppress internal conversion (IC) from the spin-converting high-lying triplet state to any lower triplet states. Kasha's rule dictates that such a process is not highly likely, and indeed, there is no direct evidence on inhibited triplet IC. Here, we suggest spin conversion in polaron pairs (PPs) as another channel that can also enhance singlet exciton generation. In our model, the spin states may interconvert by hyperfine coupling and the singlet exciton yields can be influenced by the relative rates of charge recombination of singlet and triplet PPs. We calculate the rate constants of the charge recombination, IC, ISC, and hRISC processes of hot exciton molecules and apply them to generate a kinetic picture via the kinetic master equation, toward examining changes in singlet exciton yields. The results show increases in the singlet exciton generation when the recombination within triplet PP is slower than within singlet PP and when that recombination occurs at a rate comparable to or slower than the hyperfine coupling-induced spin conversion. Additionally, correlation analyses demonstrate that although electronic coupling predominantly determines charge recombination rates, the energy barriers still contribute significantly, manifesting the need of considering both coupling and energy barriers during charge recombination processes in PPs.